scsi: megaraid_sas: Create separate functions for allocating and freeing controller DMA buffers

Code refactoring - create separate functions to allocate and free
controller DMA buffers

Signed-off-by: Kashyap Desai <kashyap.desai@broadcom.com>
Signed-off-by: Shivasharan S <shivasharan.srikanteshwara@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>